linux-kernel - Re: [RFC PATCH v2 3/6] timekeeping: Fix cross-timestamp interpolation for non-x86

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Date:   Thu, 24 Aug 2023 21:04:34 -0700
From:   John Stultz <jstultz@...gle.com>
To:     Peter Hilber <peter.hilber@...nsynergy.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        "Christopher S. Hall" <christopher.s.hall@...el.com>
Subject: Re: [RFC PATCH v2 3/6] timekeeping: Fix cross-timestamp interpolation
 for non-x86

On Thu, Aug 17, 2023 at 6:20 PM Peter Hilber
<peter.hilber@...nsynergy.com> wrote:
>
> So far, get_device_system_crosststamp() unconditionally passes
> system_counterval.cycles to timekeeping_cycles_to_ns(). But when
> interpolating system time (do_interp == true), system_counterval.cycles is
> before tkr_mono.cycle_last, contrary to the timekeeping_cycles_to_ns()
> expectations.
>
> On x86, CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE will mitigate on
> interpolating, setting delta to 0. With delta == 0, xtstamp->sys_monoraw
> and xtstamp->sys_realtime are then set to the last update time, as
> implicitly expected by adjust_historical_crosststamp(). On other
> architectures, the resulting nonsense xtstamp->sys_monoraw and
> xtstamp->sys_realtime corrupt the xtstamp (ts) adjustment in
> adjust_historical_crosststamp().
>
> Fix this by deriving xtstamp->sys_monoraw and xtstamp->sys_realtime from
> the last update time when interpolating, by using the local variable
> "cycles". The local variable already has the right value when
> interpolating, unlike system_counterval.cycles.
>
> Fixes: 2c756feb18d9 ("time: Add history to cross timestamp interface supporting slower devices")
> Signed-off-by: Peter Hilber <peter.hilber@...nsynergy.com>

Thanks again for iterating on this.  This looks much better!

Now, I've never had an environment that used this logic, so I'm
trusting you've tested it well?

Assuming so:
Acked-by: John Stultz <jstultz@...gle.com>

thanks
-john